CONCRETE DESIGN 305 



sufficient steel is inserted, this rule will of course be as safe 

 as the formula already given. 



In a large building, the loads on the columns in the lower 

 floors become very great, and as it is usually not desirable 

 to increase the outside size of the column, the percentage of 

 reinforcement is increased. A favorite style of reinforce- 

 ment employed under such conditions is shown in Fig. 4. 

 It consists of four angles riveted back to back in the center 

 of the column to form a steel core. The area of this steel 

 core can be increased if desired by using packing plates 

 between the angles. The laws of this city then assume 

 that the steel core takes all the vertical 

 load and that the concrete prevents it 

 from bending sidewise. The rule for 

 designing such a column is, therefore, to 

 allow a safe stress of 16,000 Ib. per sq. in. 

 on the steel and nothing on the concrete. 



Thus, suppose 160 T. is to be safely car- 



ried. Thisequals 160 X 2,000 = 320,000 Ib. ~~ 



The area required for the steel core, then, 



will be 320,000-^16,000 = 20 sq. in. If four angles are to 



be used, each one will have to have an area of 20 -*- 4 = 5 sq. in. 



Angles 4 in. X 4 in. X tt in. would be large enough. 



It is recommended that vertical rods be inserted in the 

 concrete around the steel core to help stiffen it. The rule 

 just given is of an empirical nature and is here presented 

 to show the practice sometimes followed. 



Hooped Reinforcement. Hooping of concrete columns 

 increases the ultimate strength of the concrete contained 

 inside the hooping, but has little effect below the elastic 

 limit. Therefore, with hooping, a greater unit stress may be 

 used, although the steel in the hooping itself does not carry 

 the load applied. The amount of hooping should not be 

 less than 1% of the volume of the concrete inclosed. The 

 clear spacing between the bands should not exceed one- 

 fourth the diameter of the inclosed column. Adequate 

 means must be provided to hold the hooping in place while 

 the concrete is being placed. The Joint Committee allows 

 a unit working stress of 540 Ib. on hooped concrete columns, 



